# CVE-2026-116270: MIME::Words memory exhaustion in decode_mimewords

Source: https://junglewise.ai/threats/cve-2026-116270-mime-words-versions-before-5-521-for-perl-allow-memory  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-28. Updated: 2026-10-28  
License: CC BY 4.0 with a link back

CVE-2026-116270 · Severity: info · Published 2026-10-28

MIME::Words is a Perl library used to parse and decode email headers. An attacker can craft a specially designed email header with repeated incomplete encoded-word sequences ("=?") that causes the decoder to accumulate error messages, exhausting available memory and potentially crashing email parsing systems or services.

Vendors: [CPAN](https://junglewise.ai/threats/vendors/cpan).

## Quick answers

**Is CVE-2026-116270 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-116270 being exploited in the wild.

**How severe is CVE-2026-116270?** Informational severity, published 28 October 2026.

**Which products and versions are affected?** CPAN MIME::Words before 5.521.

**Is there a fix for CVE-2026-116270?** Yes: fixed in 5.521. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-116270 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-116270?** Check whether you run CPAN MIME::Words at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- Fixed in 5.521

## Technical details

The decode_mimewords function in MIME::Words before version 5.521 appends the entire input string to the $@ error variable for each incomplete encoded-word sequence ("=?" without a complete encoding header). A crafted Content-Type header with 8,000 such sequences (16 KB of data) results in approximately 128 MB of error strings, causing unbounded memory growth. From MIME::Words 5.518 onwards, MIME::Parser decodes every multipart boundary through this vulnerable function, allowing remote exploitation via email headers.

## Affected products

- CPAN MIME::Words before 5.521

## Timeline

- 2026-10-28: disclosed
- 2026-10-27: patched: Version 5.521 released with fix to stop copying input into error messages

## References

- [https://cpan.org/modules](https://cpan.org/modules)
- [https://metacpan.org/release/DSKOLL/MIME-tools-5.521/changes](https://metacpan.org/release/DSKOLL/MIME-tools-5.521/changes)